Many graphical computer interfaces are being used in an effort to match more closely the established habits of humans when they communicate with writing. In the most natural and intuitive of these interfaces, a human user "writes" with a stylus on the surface of a computer-driven display, entering handwriting, sketching a diagram or picture, pressing a "button", etc. What actually happens, of course, is that the user stimulates a digitizer, co-located with the display, and the digitizer signals the position of the stylus to the computer. The computer then responds appropriately, driving the display to reproduce the path of the stylus (thus making an image of the handwriting or picture) or to signal that the button has been pressed. In this way, the stylus/digitizer/display combination becomes a close metaphor to the common pencil and paper tablet that nearly everyone uses.
However, to complete the metaphor, an erase function is needed to complement the write function. In the case of the pencil and tablet, an eraser is conveniently located opposite the writing end of the pencil, and the user merely has to invert the pencil to use it. Such simplicity should also characterize the electronic equivalent.
Several examples of providing an erase function are found in the art. In Levin et al (U.S. Pat. No. 5,231,578, issued Jul. 27, 1993), a stylus with writing and erasing ends is paired with a digitizer and the combination drives a display. However, the display is separate from the digitizer and thus some of the intuitive analogy to the pencil/tablet is missing. In addition, the stylus is an active device, tethered to the system with a cable carrying power and signals.
Farel et al (U.S. Pat. No. 4,697,050, issued Sep. 29, 1987) discloses a self-contained stylus with physical writing and erasing ends. This is used in conjunction with an actual drawing board. However, an electronic grid under the board picks up signals from the stylus as it is writing (or erasing) and sends these to a computer for recording or other use. The stylus is an active device powered with an internal battery.
Other examples exist in the art in which erasure takes place by manipulating a graphical erase icon around the display screen with a stylus. Likewise, there are examples in which the write and erase functions are carried out by separate handheld devices.
If both write and erase functions are allowed in intuitive interfaces, it appears that this leads, inevitably, to a stylus with active circuitry in order to distinguish the two functions.
It would considerably advance the art to have a simple, passive, and inexpensive stylus which can be used very much like a pencil for both writing and erasing.